Typecast to convert String to Char

So I have this palindrome program where the constructor will remove all the punctuations from string s and store it into s2. My teacher does not allow me to use the charAt method, only allowing the use of the substring method. When I try to typecast a substring to char and then int to see if the character is a letter, I get a compiler error saying it cannot cast string to char. Why?

                public Palindrome(String s)
                {
                    s1 = s;
                    for(int i = 0; i < s.length(); i++)
                    {
                        if((((int) Character.valueOf(s.substring(i, i + 1))) >= 65) && (((int) Character.valueOf(s.substring(i, i + 1))) <= 90))
                        {
                            if((((int) Character.valueOf(s.substring(i, i + 1))) >= 97) && (((int) Character.valueOf(s.substring(i, i + 1))) <= 122))
                            {
                                s2 += s.substring(i, i + 1);
                            }
                        }
                    }
                }